MX jeans aren't, some enduro ones claim to be.
I've had Sinisalo enduro jeans that were ok to a point. My current Acerbis 'Clima' pair are based on snowmobile pants, and have been pretty good. I will likely try a pair of Klim jeans next, heard a lot of good things about them.
HTH
Mark
Another vote fo Klim here but they are no more waterproof then anything else out there. I use a pair of Gortex liners from an old pair if Ralye trousers. Coupled with Seal Skins I manage to keep quite dry